Upon ordering a burger in a fast food chain, diners might have already come to terms with the fact there is a very slim chance of horse meat lurking under that tomato.

But the additions of rat and human DNA could be harder to swallow.

A new study from US-based food testing company Clear Labs has discovered, from a sample of 258 burgers, two cases of meat in vegetarian products, three burgers with rat DNA and one case of human DNA.

“The most likely cause is hair, skin, or fingernail that was accidentally mixed in during the manufacturing process,” it read, referring to the human DNA.

“What many consumers don't know is that some amounts of human and rat DNA may fall within an acceptable regulatory range,” the report added.

It also found that there are “gaps” in food safety and quality standards, but overall the beef industry has “benefited from stringent regulation and aggressive testing requirements”.

Of larger concern than the “unpleasant” DNA findings was that almost a quarter of vegetarian burgers have different ingredients to those on the label. Two veggie burgers contained beef, and one black bean burger contained no black beans.

Although Americans are unlikely to mix up their bison from their ground lamb patties on the barbecue, it seems the burger industry is making that mistake. Around 6 per cent of the burgers substituted one product for another. A common finding was that, for meat burgers, chicken or turkey was unexpectedly added in.

Another problem is that 4.3 per cent of burgers contained pathogenic DNA, which can cause food outbreaks and food poisoning.

Certain pathogens found in the products can cause tuberculosis-like symptoms, gastroenteritis, foodborne illness and E. coli.

Pathogens in four vegetarian burgers, considered a low risk category for food poisoning, was “troubling”, the report said.

Mixed-up meats, possible illness and rats aside, for those watching their weight, it is worth noting that 12 burgers in the study contained at least 100 more calories than on the label.